Since polyester fiber has many excellent properties including high tenacity, high Young's modulus and heat-resistant dimensional stability, it is used in a wide range of fields including clothing and industrial use. The physical properties of polyester fiber are known to differ significantly depending on the type of catalyst used for polycondensation of the polymer.
For example, antimony compounds are widely used as polycondensation catalysts for polyethylene terephthalate fibers because of their excellent performance as polycondensation catalysts. Also, examples of addition of specific phosphorus compounds are described in Patent document 1 and elsewhere. Patent document 2 describes examples of using reaction products of titanium compounds and phosphorus compounds.
Even when such catalysts are used, however, the effect of improvement in the spinning property is inadequate, and even more improvement in the spinning property is desirable. Lower spinning properties lead to variation in the properties of the obtained fiber, and hence a demand exists for higher performance polyester fiber.
Incidentally, it has become common in recent years to use cords made of polyester fibers, as carcass materials in radial tires for passenger vehicles. This is because the polyester fibers are even lower cost materials, with an excellent balance of properties including tenacity, modulus and dimensional stability, compared to the conventionally used nylon fibers and rayon fibers (Patent document 3 and Patent document 4). One type of polyester fiber that has been developed is HMLS (High Modulus-Low Shrinkage) polyester fiber, obtained by high-speed spinning and having excellent dimensional stability and an excellent modulus, and using HMLS polyester fiber as the carcass material in radial tires for passenger vehicles can contribute to improved maneuvering stability of the tires, as well as increased uniformity.
On the other hand, with the increasing need to reduce the environmental load associated with automobiles and raise fuel efficiency in recent years, the goal of low fuel efficiency is being pursued through lower tire rolling resistance and lighter weight, in addition to the aforementioned targets of improved maneuvering stability and greater uniformity. Methods for reducing tire weight, for example, include reducing the amount of rubber used in tires and reducing the amount of tire reinforcing material used. Even in such cases, however, still further improvement in durability and dimensional stability is desired for the carcass materials composed mainly of tire-supporting fiber. In addition, the carcass material and the actual reinforcing fiber cords composing it generate heat due to repeated stress and strain input during tire running, and this causes increased rolling resistance.
In contrast, Patent document 3 discloses a polyester fiber cord comprising polyethylene terephthalate having 0-10 mol % of isophthalic acid as an acid component and 0-10 mol % of butylene glycol and/or propylene glycol as an alcohol component copolymerized at a total of 1-10 mol %, as a polyester fiber cord that improves the maneuvering stability and the uniformity of radial tires. Copolymerization of the polyester in this method can reduce contraction of the filaments, but since copolymerization results in a lower degree of crystallinity of the fiber and larger amorphous regions in the fiber, the heat resistance and durability are lowered during cure molding and tire running, in which it is exposed to a high-temperature atmosphere.
The polyester fiber not only has excellent high strength, dimensional stability and durability, but its general utility renders it widely applicable as a low cost material for various industrial materials, including rubber reinforcement. In recent years, in particular, increasingly superior performance including reduced weight, higher energy efficiency and increased durability is being demanded for materials because of the worldwide awareness of the need to reduce environmental load. For example, polyester fibers for carrying belts such as V-belts and conveyor belts must exhibit a higher modulus and improved dimensional stability, as well as longer durability.
In this context, there have been disclosed techniques for improving the durability of polyester fibers for belts, wherein the special metal catalysts in the polyester composing the polyester fiber are modified, or copolymerizing components and antioxidants are added (see Patent document 5 and Patent document 6, for example).
However, addition of third components in these techniques can lead to reduced strength or a lower modulus, and often sufficient dimensional stability or durability cannot be achieved for the belt reinforcing fibers. The cost is also high, presenting an economical problem. Thus, belt reinforcing fibers exhibiting sufficient performance have not been obtainable in the prior art.